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The carbon-13 nuclear magnetic resonance (13C NMR) spectra of luteinizing hormone-releasing hormone (LH-RH) and lower homologous peptides have been assigned in aqueous solutions at various pH values. 13C spin-lattice relaxation times (T1) have been measured for all proton-bearing carbons at 25.2 and 67.9 MHz. From the T1 data the rates of overall molecular motion and intramolecular motion of side chains have been estimated. LH-RH is a flexible molecule in solution, having segmental motion along the backbone as well as in the nonaromatic side chains.

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Nuclear magnetic resonance (NMR) of 13C is used to probe dynamical properties of [2-[2-13C]glycine]-, [3-[2-13C]glycine]methionine-enkephalin, and a-endorphin. Spin-lattice relaxation times (TI) are reported at 20 MHz for the enriched enkephalin samples as functions of pH, temperature, and concentration. Relaxation times are also reported for enkephalin and a-endorphin containing I3C in natural abundance. The relaxation times are used to monitor the relative flexibility of the carbon atoms in the peptides. The results imply that in enkephalin the [3-glycine] residue is more reoverall conformational properties of the enkephalins. In solution, the enkephalins appear to prefer a folded conformation, although there is some controversy about the exact nature of the folded structure (Jones et al., 1977). Published 13C N M R TI values on enkephalin suggest that the two glycine residues are more flexible than the phenylalanine residue. In order to elucidate this aspect further, we have synthesized [2-[2-~3C]glycine]methionine-enkephalin and [3-[2-13C]glycine]methionine-enkephalin. Specific enrichment permits the measurement of accurate T I values for the [2glycine] and [3-glycine] (Y carbons which show partial overlap in the 20 MHz 13C spectra. Accurate T I values allow comparison with those obtained for the [4-phenylalanine] and[ 1 -tyrosine] residues, Furthermore, the use of I3C-enriched material permits studies at lower concentrations (over two orders of magnitude) and thus detection of effects due to molecular aggregation. The enriched samples are more suitable

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C nuclear magnetic resonance spectroscopy is used to gain information on the flexibility of the backbone in peptide hormones and peptide hormone analogs. 13C spin-lattice relaxation times (TI) were measured on luliberin, the luteinizing-hormone-releasing hormone and de~(Gly-NH2)'~-luliberin-N-ethylamide in aqueous solution at 25.2 and 67.9 MHz at temperatures of 32 ', 40 ' and 55 "C. The 13C spin-lattice relaxation times indicate increased flexibility of the peptide backbone in the immediate environment of glycyl residues in luliberin ( < Glu-His-Trp-Ser-Tyr-Gly-Leu-ArgPro-Gly-NH2) and the hormone analog de~(Gly-NH2)'~-1uliberin-N-ethylamide ( < Glu-His-TrpSer-Tyr-Gly-Leu-Arg-Pro-NH-CH2CH3) in aqueous solutions. 13C NMR spectroscopy is shown to be a sensitive technique for monitoring the time-averaged conformational flexibility of peptides in solution. Activation energies (E,) of about 25 kJ/mol were obtained for rotational reorientation of non-terminal a-carbons in the peptide backbone. Rotation of methyl groups was characterized by an E, of 9.6 kJ/mol whereas reorientation of the N-terminal pyroglutamyl residue showed an E, value of 14.6 kJ/mol. The E, values of individual carbons in the side-chains of prolyl, arginyl and leucyl residues in the peptides were similar to those obtained for the a-carbon of the same amino acid residue in the peptide backbone of the hormones. 13In the course of I3C nuclear magnetic resonance studies on peptides and peptide hormones we have occasionally observed longer spin-lattice relaxation times ( T , ) for the a-carbon of glycine than for the a-carbons of optically active amino acids. In particular, studies on luliberin, the luteinizing-hormone-releasing hormone ( < Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-ProGly-NH2) revealed NTI values (where N is the number of protons directly bonded to a given carbon) for the glycyl residues which were longer than those of the a-carbons of the optically active residues. This is not unexpected for the glycyl residue in position 10 as segmental motion of the backbone will result in increased flexibility of the chain termini. The NTl values observed for glycine-6 were also longer than those of adjacent residues. and similar trends were observed. At 67 MHz, however, the so-called 'extreme narrowing' condition is not fulfilled for the relaxation times observed in luliberin and interpretation of Tl values in terms of molecular flexibility is not direct.In order to circumvent these difficulties we have studied an analogue of luliberin in which the C-terminal glycinamide residue has been replace by an ethylamide group. This modification to de~(Gly-NH2)'~-1uliberin-N-ethylamide ( < Glu-His-Trp-Ser -Tyr-Gly-Leu-Arg-Pro-NH-CH2-CH3) allows study of the glycyl-6 resonances without interference from other resonances. We have obtained the 13C chemical shifts and TI values of this analogue at 25 MHz as well as at 67 MHz at temperatures of 32, 40 and 55 "C. We have found that the a-carbon of the glycyl residue in de~(Gly-NH2)'~-luliberin-IVethylamide shows more flexi...